Non-digital inks and printing elements suitable for Magnetic Ink Character Recognition (MICR) printing are generally known. The two most commonly known technologies are ribbon-based thermal printing systems and offset technology. For example, U.S. Pat. No. 4,463,034 discloses heat sensitive magnetic transfer element for printing MICR, comprising a heat resistant foundation and a heat sensitive imaging layer. The imaging layer is made of ferromagnetic substance dispersed in a wax and is transferred on a receiving paper in the form of magnetic image by a thermal printer which uses a ribbon. U.S. Pat. No. 5,866,637 discloses formulations and ribbons which employ wax, binder resin and organic molecule based magnets which are to be employed for use with a thermal printer which employs a ribbon. MICR ink suitable for offset printing using a numbering box are typically thick, highly concentrated pastes consisting for example in about over 60% magnetic metal oxides dispersed in a base containing soy-based varnishes. Such inks are, for example, commercially available at Heath Custom Press (Auburn, Wash.). Digital water-based ink-jet inks composition for MICR applications using a metal oxide based ferromagnetic particles of a particle size of less than 500 microns are disclosed in U.S. Pat. No. 6,767,396. Water-based inks are commercially available from Diversified Nano Corporation (San Diego, Calif.).
The present embodiments are directed to curable inks. Curable inks generally comprise at least one curable monomer, a colorant, and a radiation activated initiator that initiates polymerization of curable components of the ink. In particular, the curable ink is a ultraviolet (UV) curable liquid or gel ink. In particular, the curable ink is an ultraviolet (UV) curable ink. These UV curable liquid or gel ink compositions can be used for ink jet printing in a variety of applications. In addition to providing desirable ink qualities, the present embodiments are directed to magnetic inks for use in specific applications. The curable UV ink of the present embodiments comprises magnetic nanoparticles that are coated with polymeric materials to prevent the exposure of the nanoparticles to oxygen.
UV curable gel inks are known. They are for example disclosed in, for example, U.S. Pat. Nos. 7,153,349, 7,259,275, 7,270,408, 7,271,284, 7,276,614, 7,279,506, 7,279,587, 7,293,868, 7,317,122, 7,323,498, 7,384,463, 7,449,515, 7,459,014, 7,531,582, 7,538,145, 7,541,406, 7,553,011, 7,556,844, 7,559,639, 7,563,489, 7,578,587, 7,625,956, 7,632,546, 7,674,842, 7,681,966, 7,683,102, 7,690,782, 7,691,920, 7,699,922, 7,714,040, 7,754,779, 7,812,064, and 7,820,731, the disclosures of each of which are totally incorporated herein by reference. UV curable gel inks can exhibit desirable characteristics such as improved hardness and scratch-resistance and improved adhesion to various substrates. Curable gel inks can also exhibit advantages in that dot spread of the ink can be controlled, the ink does not bleed excessively into the substrate, including porous substrates.
The present embodiments are directed to UV curable magnetic inks which comprise polymer coated metal nanoparticles. These magnetic inks are required for specific applications such as Magnetic Ink Character Recognition (MICR) for automated check processing and security printing for document authentication. One of the inherent properties of uncoated magnetic metal nanoparticles which precludes their use in the fabrication of commercial inks is their pyrophoric nature; uncoated (bare) magnetic nanoparticles of a certain size, typically in the order of a few tens of nanometers or less, ignite spontaneously when exposed to oxygen in the ambient environment. As such, uncoated magnetic metal nanoparticles are a serious fire hazard. As such, large scale production of the UV curable inks comprising such particles is difficult because air and water need to be completely removed when handling the particles. In addition, the ink preparation process is particularly challenging with magnetic pigments because inorganic magnetic particles are incompatible with organic base components.
Thus, while the disclosed curable ink formulation provides some advantages over the prior formulations, there is still a need to achieve a formulation that not only provides the desirable properties of a curable ink and a curable liquid or gel UV ink in particular, but it is also magnetic. Furthermore there is a need to achieve a magnetic curable ink formulation that is easily produced and derived from components that do not require special handling conditions.